My DS7 is this way too. I've seen this type of thing all over this website, so I'm guessing that it might be a 'gifted thing'. He has been accelerated 3 years in math. He eats up the curriculum - doesn't struggle a bit (actually it is starting to get boring for him). BUT when it comes to something simple like 14-9, he has to work it out in his head. It takes a few seconds sometimes. Sometimes he is still counting on his fingers!